Real time clock modules have various applications. They can be used for providing timing information for operating systems, for enforcing policies for time-sensitive data, for assisting in positioning calculations and the like. U.S patent application serial number 2002/0083284 of Matsubara et al., titled “Data reproduction system, data recorder and data reader preventing fraudulent usage by monitoring reproducible time limit”, U.S. Pat. No. 5,920,727 of Kikinis et al., titled “Timer-controlled computer system shutdown and startup”, U.S patent application serial number 2004/0128528 of Poisner titled “Trusted real time clock” and U.S patent application serial number 2004/0225439 of Gronemeyer, titled “Method and apparatus for real time clock (RTC) brownout detection, illustrate some usages of real time clocks.
Real time clock signals can be tampered for various reasons including copyrighting piracy, concealing hacking or tampering attempts, reducing the functionality of a device and the like.
One tampering method involves repetitive alterations of the supply voltage provided to the real time clock module, in order to force the secured real time clock (SRTC) module to reset, to get stuck, to output a secured key stored in the SRTC module, to read invalid commands, and the like.
The supply voltage can be provided by a so-called “external” voltage supply unit—a voltage supply unit that is located outside an integrated circuit that includes the secured real time clock module.
Monitoring of these external voltage supply units can be power consuming and during low power modes the monitoring should be stopped.